[TCP]: Limit processing lost_retrans loop to work-to-do cases

This addition of lost_retrans_low to tcp_sock might be
unnecessary, it's not clear how often lost_retrans worker is
executed when there wasn't work to do.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 15 insertions, 3 deletions

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 9ff456e8d6c7..c5b94c1a5ee2 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -348,6 +348,8 @@ struct tcp_sock {
         int     lost_cnt_hint;
         int     retransmit_cnt_hint;
 
+        u32     lost_retrans_low;       /* Sent seq after any rxmit (lowest) */
+
         u16     advmss;         /* Advertised MSS                       */
         u16     prior_ssthresh; /* ssthresh saved at recovery start     */
         u32     lost_out;       /* Lost packets                 */
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index d5e0fcc22a3b..0a42e9340346 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1112,7 +1112,8 @@ static int tcp_is_sackblock_valid(struct tcp_sock *tp, int is_dsack,
  *
  * Search retransmitted skbs from write_queue that were sent when snd_nxt was
  * less than what is now known to be received by the other end (derived from
- * SACK blocks by the caller).
+ * SACK blocks by the caller). Also calculate the lowest snd_nxt among the
+ * remaining retransmitted skbs to avoid some costly processing per ACKs.
  */
 static int tcp_mark_lost_retrans(struct sock *sk, u32 received_upto)
 {
@@ -1120,6 +1121,7 @@ static int tcp_mark_lost_retrans(struct sock *sk, u32 received_upto)
         struct sk_buff *skb;
         int flag = 0;
         int cnt = 0;
+        u32 new_low_seq = 0;
 
         tcp_for_write_queue(skb, sk) {
                 u32 ack_seq = TCP_SKB_CB(skb)->ack_seq;
@@ -1151,9 +1153,15 @@ static int tcp_mark_lost_retrans(struct sock *sk, u32 received_upto)
                                 NET_INC_STATS_BH(LINUX_MIB_TCPLOSTRETRANSMIT);
                         }
                 } else {
+                        if (!new_low_seq || before(ack_seq, new_low_seq))
+                                new_low_seq = ack_seq;
                         cnt += tcp_skb_pcount(skb);
                 }
         }
+
+        if (tp->retrans_out)
+                tp->lost_retrans_low = new_low_seq;
+
         return flag;
 }
 
@@ -1481,8 +1489,8 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
                 }
         }
 
-        if (tp->retrans_out && highest_sack_end_seq &&
-            after(highest_sack_end_seq, tp->high_seq) &&
+        if (tp->retrans_out &&
+            after(highest_sack_end_seq, tp->lost_retrans_low) &&
             icsk->icsk_ca_state == TCP_CA_Recovery)
                 flag |= tcp_mark_lost_retrans(sk, highest_sack_end_seq);
 
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 53296753b0bd..324b4207254a 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1914,6 +1914,8 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
                                 printk(KERN_DEBUG "retrans_out leaked.\n");
                 }
 #endif
+                if (!tp->retrans_out)
+                        tp->lost_retrans_low = tp->snd_nxt;
                 TCP_SKB_CB(skb)->sacked |= TCPCB_RETRANS;
                 tp->retrans_out += tcp_skb_pcount(skb);